According to the International Cosmetic Ingredient Dictionary and Handbook, Tenth Ed., 2004 (herein incorporated by reference in its entirety) hair colorants (e.g., materials which impart color to hair) are classified as temporary, semi-permanent, permanent or progressive.
Temporary hair colorants include color rinses, which provide color that lasts until the first shampooing. Ingredients which impart temporary color may have a fairly high molecular weight and are unable to penetrate the hair shaft. These materials are simply deposited onto the hair fiber and are removed by subsequent shampooing.
Direct dyes generally provide color through several shampooings. These materials are either low molecular weight pre-formed colored compounds that can penetrate the hair shaft to some extent or larger molecular weight colored compounds that interact with the surface of the hair fiber via non-covalent interactions.
Oxidative dyes are considered permanent hair coloring preparations as they provide color that is not substantially removed by shampooing. Oxidative coloration involves the deposition of an essentially colorless intermediate and of a coupling agent inside the hair fiber. In the presence of an oxidizing agent, usually hydrogen peroxide, these chemicals form a series of complex colored dyestuffs inside the hair fiber. Progressive hair coloring preparations are oxidative hair colors which develop color gradually by repeated applications.
While oxidative dyes may be preferred due to their long-lasting coloration, they have some disadvantages to direct coloring preparations. For example, direct dyes are more vibrant and provide a more “pure” color than do the oxidative dyes. It will be appreciated by those skilled in the art that the “pure” color of direct dyes is due to generally higher extinction coefficients and narrower peak widths at half height than oxidative dyes. Oxidative dyes can also participate in side reactions during the application process that can affect the final hair color and perform less well on damaged hair. Therefore, it would be desirable to develop non-oxidative dyes with the color and vibrancy of a direct dye and the long lasting color fastness of an oxidative dye that optionally performs better on damaged hair than on non-damaged hair.